The magnetic disk drive is for several reasons the most common of the mass data storage devices used in computing systems today. Those drives having rigid disks as the medium have relatively large capacity, a price per bit stored that is quite low and rapid availability of the data stored. The rapid availability of the data is due mainly to the high rotational speed of the rigid disks, which in turn is dependent on the use of aerodynamically suspended flyers carrying the data transducers a few microinches above the disk surface. The surfaces of the flyers facing the disks are designed to create aerodynamic lift resulting from the disk's rotation which, when the flyer is properly suspended from an arm, causes the trailing edge of the flyer to accurately follow the surface of the disk just a few microinches from it. The arm may be moved to shift the flyer across a portion of the radius of the disk, thereby allowing the transducer to read data previously written on the disk in a magnetic medium layer and to write data in the medium layer.
To reliably fly the flyer for the years of failure-free performance now expected for such disk drives, it is important that the disk be as flat and free of imperfections as is possible. However, since it is not possible to have disks that have totally perfect surfaces, or for that matter spindle bearings with no axial runout or vibration, the flyers themselves are suspended by a gimbal mounting from a flexible arm. The arm and gimbal cooperate to allow the flyers to adjust themselves to the changes in the attitude of the disk surface. Thus, vertical (axial) runout of the disk surface or asperities in the disk surface will result in the flyer adjusting itself to the changing surface attitude and altitude rather than causing the flyer and disk surface to contact and possibly be damaged, i.e. "crash". Vertical disk runout refers to relatively gradual changes in the position of the disk normal to its surface. The term "asperity" in this context means a relatively localized projection on the surface of a disk on which a flyer suitable for carrying a transducer may be flown, which projection is of a height and size as to test the tolerance of the flyer and its subsystem to such defects when the flyer passes over it during normal flying operation. For convenience' sake, the flyer or slider, the gimbal which attaches the slider to the arm, and the flexible arm itself, will on occasion be referred to as the SGA (slider-gimbal-arm) assembly hereafter.
The flexible arm and the gimbal suspension form a delicate subsystem of the drive for supporting the flyer. During manufacture of these parts one must be certain that the characteristics of each suspension and arm will permit them to properly support the flyer over the entire life of the drive. Consequently, it is important that samples of the arm/suspension subsystem are tested to make sure that their dynamic response to the various disk surface defects and conditions is satisfactory, thus insuring that the manufacturing processes for the subsystem have not degraded. In one type of such testing the flyers are actually flown on test disks which have surface defects artificially placed on them.
In the past the desired asperities on test disks have been created by either distorting the disk surface in a predetermined manner or by depositing a material to create a number of individual asperities on the disk surface. The former procedure is exemplified by IBM Technical Disclosure Bulletin , Vol. 25, No. 3A, August 1982, p. 1087, "Calibrated Asperity Disk" (Broome), where the asperities are created by creating convex bumps with carefully controlled height in the surface of the disk by drilling through the disk from the bottom side. The latter procedure is described in IBM Technical Disclosure Bulletin , Vol. 21, No. 12, May 1979, "Asperity Disk" (Blair, et al.), and in IEEE Transactions on Magnetics , Vol. MAG-18, No. 6, November 1982, "A Standard Disk for Calibrating Head-Disk Interference Measuring Equipment" (Mackintosh et al.). The latter references suggest the use of any of a number of materials to constitute the protrusions. Among the materials mentioned are chromium, various metal carbides, and tungsten, selected for resistance to wear. The harder of these materials can damage the flyer under test. The softer ones have been determined to not have sufficient wear resistance. It is important that such test disks have asperities that are durable and whose characteristics can be reproduced with accuracy, so that testing over a period of time has a standardized basis which will indicate quickly when the arm/suspension subsystem under test is faulty. If the test disk in use is or can possibly be the source of the change in test results, no worthwhile data will be generated defining the characteristics of the arm/suspension subsystem and flyer under test.
It frequently is desirable to place asperities of controlled height and footprint size and shape on a transparent glass disk, so that performance of the flyer as it passes over them can be directly observed. For example, if the area is illuminated with monochromatic light, the Moire interference fringes will give a good indication of the changes in the flying height. Unfortunately, the materials usually selected to constitute the asperities adhere very poorly to glass. Accordingly, it has been difficult in the past to directly observe the dynamic response of disk memory flyers when encountering during flying, asperities whose height and footprint dimensions are accurately controlled.